Conventional aircraft propeller systems typically incorporate a plurality of variable pitch propeller blades mounted to a rotary hub driven by the aircraft's engine, with each propeller blade extending radially outwardly from the hub along the longitudinal axis of the blade. In order to permit pitch adjustment, each blade is mounted to the hub for pivotable movement about its longitudinal axis. The hub typically encloses a chamber within its interior wherein a pitch change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch of the blades thereby altering air resistance to the rotation of the blades to thereby control engine speed.
Generally, the actuation system includes a pitch change actuator of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of the hydraulic fluid which drives the actuator. The adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by way of collateral apparatus, a change in pitch change fluid pressure whenever the monitored engine speed departs from the desired engine speed setting. To control blade pitch, the net pressure force exerted by the pitch change fluids selectively directed in response to a departure from desired engine speed against the opposite faces of the piston, that is the difference between the pressure force exerted by the fine pitch change fluid on one face of the piston and the pressure force exerted by the coarse pitch change fluid on the opposite face of the piston, is varied thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston.
Typically, the fine and coarse pitch change fluids are delivered through independent conduits in an axially elongated tube assembly to opposite sides of the pitch change piston. For example, the fine pitch change fluid is delivered to a fine pitch fluid chamber adjacent the forward face of the pitch change piston and the coarse pitch change fluid to a coarse pitch fluid chamber adjacent the rearward face of the pitch change piston. The fluid delivery tube assembly, typically referred to as a torque tube, commonly comprises a pair of co-axially disposed tubes forming an annular fluid delivery conduit therebetween opening to one of the fluid chambers and an inner conduit within the interior of the inner tube opening to the other fluid chamber, the fine pitch change fluid being delivered through one of these conduits and the coarse pitch change fluid through the other conduit. The inner tube is mounted at its forward end to the propeller hub and the outer tube is mounted at its forward end to the pitch change piston whereby the outer tube not only rotates with the propeller hub but also translates axially with the pitch change piston, while the inner tube rotates with the propeller hub but does not translate with the pitch change piston.
In order to provide a feedback signal indicative of blade pitch setting to the controller that selectively meters the pitch change fluids, it is common practice to monitor the movement of the translating fluid delivery tube since this tube is attached to the pitch change actuator piston and moves therewith. It is well known in the art to utilize a linear variable differential transformer (LVDT) of conventional sliding armature/surrounding coil construction as a means of generating such a feedback signal indicative of the position of the translating tube of the fluid delivery assembly. Customarily, a pair of independent LVDT's are used to provide redundant feedback signals, each LVDT disposed axially parallel to the translating tube with its core mounted to the distal end a spring loaded shaft which in turn is operatively attached at its other end to the translating outer fluid delivery tube so as to translate therewith whereby the core reciprocates within a stationary cylinder housing the LVDT coils. The stationary cylinder is typically mounted to the non-translating tube of the fluid delivery assembly and houses a pair of axially spaced secondary coils and a primary coil disposed centrally therebetween. As the pitch change piston moves axially in response to a change in blade pitch, the translating outer fluid delivery tube will correspondingly move axially and the core of the LVDT will slide within the stationary cylinder, thereby causing the voltages induced in the secondary coils to change responsively. The difference between the voltages induced in the axially spaced secondary coils is indicative of the displacement of the core from its null position, i.e. a central position between the two axially spaced secondary coils. Thus, the LVDT measures the stroke of the pitch change piston and provides a feedback signal which is indicative of blade pitch setting.
As well appreciated in the art, it is desirable to provide an alarm signal to the controller whenever the propeller blades are positioned at an undesirably low pitch angle. In flight, the forces acting on the propeller blades are transmitted to the pitch change actuator and tend to drive the blades to a lower pitch angle unless balanced by a counteracting net pressure force of sufficient magnitude. Under normal circumstances, the controller is able to respond to the LVDT feedback signal indicative of blade position so as to modulate the coarse pitch pressure, or both the coarse pitch pressure and the fine pitch pressure, so as to increase the magnitude of the counteracting net pressure to compensate for the increase in blade loading in order to maintain the blades at a desired pitch setting. In the event of a failure or malfunction in the control system, for example such as a loss of hydraulic pressure, resulting in the inability of the controller to increase the net pressure force sufficiently to counteract the increased blade loading, the propeller blades will migrate to a finer pitch setting and, potentially, to an undesirable overspeed condition.
Accordingly, it is a common practice in the prior art to provide a mechanical low pitch switch in operative association with the torque tube which transmits an alarm signal to the controller in the event that the torque tube has translated beyond a preselected point in the fine pitch direction. Typically, the mechanical low pitch switch comprises a microswitch having an actuation lever which includes a wheel mounted at its distal end that rides on a cam member disposed about an anti-rotation rod for axial translation therealong. The cam member is mounted on an outer sleeve of a bearing assembly disposed about the torque tube such that the cam member translates with the torque tube but does not rotate with the torque tube. If the pitch change actuator is translated in a fine pitch direction under increasing blade loading beyond the preselected limit, the cam member translates with the torque tube to which it is mounted in the fine pitch direction and the wheel at the distal end of the actuation lever drops of the end of the cam member when the pitch change actuator passes beyond the preselected limit, thereby activating the microswitch to transmit a low pitch alarm signal to the controller to initiate emergency corrective action.
Conventional mechanical microswitches exhibit relatively low sensitivity as low pitch alarm switches. Thus, it is customary to build in a high margin of safety by designing the switch to activate at a pitch angle greater than the true low pitch limit. Furthermore, the bearing assembly and anti-rotation structure which is necessitated to ensure that the cam member with which the switch lever is operatively associated does not rotate with the torque tube, results in a cumbersome and unduly complicated mechanical system which requires frequent servicing and realignment checks.